The present invention relates to a semiconductor device and, more specifically, a semiconductor device having an output transistor using low leak current.
There is a known configuration of generating a clock signal to be supplied to a real-time clock by a quartz oscillation circuit comprised of an amplification circuit provided in a microcomputer and a quartz oscillator coupled to an external terminal of the microcomputer. On the other hand, in the case where a quartz oscillator is not coupled to an external terminal due to limitation of the number of external terminals of a microcomputer (in the case where the function of a real-time clock is unnecessary), it is desired to use the external terminal also as an input/output terminal of an I/O port.
In the case where a quartz oscillator is coupled to an external terminal, the influence of an I/O port formed in the microcomputer is a concern. In this case, input/output buffers as components of the I/O port are inactive (in a high-impedance (H-Z) state). There is a concern that leak current of the output buffer disturbs normal operation of the quartz oscillation circuit. Japanese Unexamined Patent Publication No. 2004-104759 (patent literature 1) discloses the configuration of a semiconductor device in which gate leak current of a driver circuit having a transistor whose gate oxide film is thin is suppressed. In the case of holding a conductive state of an output transistor coupled to an output terminal, by making on voltage which is applied across the source and the gate of the output transistor lower than power source voltage, gate leak current is suppressed.